Uniform, patterned abrasive grain placement on various categories of abrasive tools has been found to improve abrasive tool performance. One such category of tools, the “engineered” or “structured” coated abrasive tools designed for fine, precision grinding operations, has become commercially available over the past decade. Typical designs for these coated abrasive tools are described in U.S. Pat. Nos. A-5,014,468, A-5,304,223, A-5,833,724, A-5,863,306 and 6,293,980B. In these tools, small, shaped composite structures, e.g., three-dimensional pyramids, diamonds, lines and hexagonal ridges, containing a plurality of abrasive grains held within bond material, are replicated as a single layer in a regular pattern on the surface of a flexible backing sheet. These tools have been found to engage in freer cutting, and the open spaces between the grain composites allow for cooler grinding and enhanced debris removal. Similar tools in the superabrasive tool category, having a rigid, shaped backing disc or core, are disclosed in U.S. Pat. No. 6,096,107.
Abrasive tools have been designed having a single layer of abrasive grains laid out in a uniform grid pattern of squares, circles, rectangles, hexagons, or other replicated geometric patterns and these tools have been used in a variety of precision finishing applications. A pattern may comprise individual grains or parcels of abrasive grains in a single layer, separated by open spaces between the parcels. Particularly among superabrasive tools, uniform patterns of abrasive grains are thought to render more planar, smoother surface finishes than can be achieved with random placement of abrasive grains on the abrasive tool. Such tools are disclosed, for example, in U.S. Pat. Nos. 6,537,140B1, A-5,669,943, A-4,925,457, A-5,980,678, A-5,049,165, 6,368,198B1 and A-6,159,087.
Thus, various abrasive tools have been designed and manufactured according to highly precise specifications required for the uniform abrasion of costly semi-finished workpieces. As an example of such workpieces in the electronics industry, semi-finished integrated circuits must be abraded or polished to remove excess ceramic or metal materials that have been selectively deposited in multiple surface layers, with or without etching, onto wafers (e.g., silica or other ceramic or glass substrate material). The planarization of newly formed surface layers on the semi-finished integrated circuits is done with chemical mechanical planarization (CMP) processes using abrasive slurries and polymeric pads. The CMP pads must be continuously or periodically “conditioned” with an abrasive tool. Conditioning eliminates pad hardening or glazing caused by the compression of accumulated debris and abrasive slurry particles into the polishing surface of the pads. The conditioning action must be uniform across the surface of the pad so that the conditioned pad once again can planarize the semi-finished wafers across the entire surface of the wafers.
The location of abrasive grains on the conditioning tool is controlled to effect uniform scratch patterns on the polishing surface of the pad. Fully random placement of abrasive grain on a two-dimensional plane of the tool generally is considered unsuitable for CMP pad conditioning. It has been suggested to control the location of abrasive grains on CMP conditioning tools by orienting each grain along some defined uniform grid on the abrading surface of the tool. (See, for example, U.S. Pat. No. 6,368,198 B1.) However, uniform grid tools have certain limitations. For example, a uniform grid gives rise to a periodicity in vibration arising from the tool movement that, in turn, can cause waviness or periodic grooves on the pad or uneven wear of the abrasive tool or of the polishing pad, ultimately translating to inferior surfaces on the semi-finished workpiece.
A method for creating a non-uniform grid pattern of abrasive grains in a single layer on an abrasive tool substrate is disclosed in JP Pat. No. 2002-178264. In making these tools, one begins by defining a virtual grid having a uniform, two-dimensional pattern, such as a series of squares, wherein grains are to be placed at the intersections of lines on the grid. Then, one randomly selects some intersections along the grid and displaces grains from these intersections, moving the grains a distance of less than three times the average grain diameter. The method makes no provision for insuring the placement of individual grains in a numerical sequence along the x or y axis, thus failing to insure that the resultant tool surface can deliver consistent abrading action, without significant gaps or inconsistencies in the area of contact when the tool traces a linear path over a workpiece. The method also fails to insure a defined exclusionary zone around each abrasive grain, thus permitting both zones of concentrated grains and zones with gaps between grains that can cause non-uniform surface qualities in the finished workpiece.
Having none of these deficiencies of JP Pat. No. 2002-178264, the present invention permits one to manufacture abrasive tools having a defined exclusionary zone around each abrasive grain in a random, but controlled, two-dimensional array. Further, tools can be manufactured having a randomized numerical sequence of abrasive grain locations along the x and/or y axis of the grinding surface of the tool so as to create consistent abrading action, without significant gaps or inconsistencies in the area of contact, as the tool traces a linear path over the workpiece.
Prior art abrasive tools made with a uniform grid array of grains arranged by placing individual abrasive grains into interstitial voids of a template wire screen or perforated sheet (e.g., as in U.S. Pat. No. A-5,620,489) are limited to the static, uniform structural dimensions of such a grid. These wire screens and uniformly perforated sheets only can produce a tool design having a grid of regular dimensions (often a square or diamond grid). In contrast, tools of the invention may employ non-uniform distances, in a variety of lengths, between abrasive grits. Thus, vibration periodicity may be avoided. Freed from template screen dimensions, the cutting surface of the tool may contain a higher concentration of abrasive grain and may employ much finer abrasive grit sizes while still controlling grain placement. For CMP pad conditioning, it is believed that the higher the concentration of abrasive grains on the abrasive tool, the greater the number of abrasive points in contact with the pads and the higher the efficiency of removal of accumulated oxide debris and other glazing materials from the polishing surface of the pads. Because CMP pads are relatively soft, small abrasive grit sizes are suitable for use in this application and one may use relatively higher concentrations of a smaller grit size abrasive grain.
Furthermore, in peripheral grinding operations carried out with the tools of the invention, each grain in the controlled, random array of non-contiguous abrasive grains will trace different, self-avoiding paths or lines along the surface of the workpiece as it moves in a linear fashion. This contrasts favorably with prior art tools having a uniform grid array of abrasive grains. In a uniform grid, each grain sharing the same x or y dimension on the grid will trace along the surface of the workpiece in the same path or line traced by all other grains lying at the same x or y dimension which also traverse the pad. In this manner, the uniform grid tools of the prior art tend to create “trenches” on the surface of the workpiece. The tools of the invention minimize these problems. Tools operated in a rotary fashion rather than in a linear fashion present a different situation. With a “face” or surface grinding tool, regular arrays of grain have multi-fold rotational symmetry, (e.g., a square uniform grid has a four-fold rotational symmetry, hexagonal has six-fold, etc.) whereas the tools of the invention have only one-fold rotational symmetry. Thus, the repeat cycle of the tools of the invention is much longer (e.g., 4 times longer than a square, uniform grid) with the net effect that the tools of the invention minimize the creation of regular patterns on the workpiece, relative to tools having a regular uniform array of abrasive grain.
In addition to benefits realized in peripheral grinding and CMP pad conditioning, the abrasive tools of the invention offer benefits in various manufacturing processes. These processes include, for example, abrading other electronic components, e.g., backgrinding ceramic wafers, finishing optical components, finishing materials characterized by plastic deformation and grinding “long chipping” materials, e.g., titanium, Inconel alloys, high tensile steel, brass and copper.
While the invention is particularly useful in making tools having a single layer of abrasive grain on a planar work surface, a two-dimensional grain array may be bent or formed into a hollow three-dimensional cylinder and thereby adapted for use on tools constructed as a cylindrical three-dimensional array of abrasive grain held on the surface of the tool (e.g., rotary dressing tools). The abrasive grain array may be converted from a two-dimensional sheet or structure to a solid, three-dimensional structure by rolling the sheet bearing the bonded abrasive grain array into a concentric roll, thus creating a spiral structure in which each grain is randomly offset from each adjacent grain in the z direction and all grains are non-contiguous in the x, y and z direction. The invention also is useful in making many other sorts of abrasive tools. These tools include, for example, surface grinding disks, edge grinding tools comprising a rim of abrasive grain around the perimeter of a rigid tool core or hub, and tools comprising a single layer of abrasive grain or abrasive grain/bond composite on a flexible backing sheet or film.